Improved strength and ductility balance of medium-carbon steel with chromium and titanium fabricated by friction stir welding process

To investigate the effect of alloying elements on the microstructure and mechanical properties of medium-carbon steel weldments, 0.4% carbon steels with different chromium and titanium contents were fabricated by melting and hot-rolling for friction stir welding (FSW). The microstructures in the stir zones of friction stir welded (FSWed) medium-carbon steels without chromium, with 1% chromium and 4% chromium are ferrite and pearlite, bainite, and fully lath martensite, respectively. The prior austenite grain size was refined by the addition of titanium. High densities of ultrafine-twins are observed in the martensite of FSWed 0.4%C–4%Cr and 0.4%C–4%Cr-0.2%Ti steels, while dense auto-tempered iron carbides with less ultrafine-twins are observed in the lath martensite of FSWed 0.4%C–4%Cr-0.4%Ti steel. The strength of FSWed 0.4%C–4%Cr steels is improved by the martensitic structure, while the ductility of 0.4%C–4%Cr steels decreases significantly compared to that of 0.4%C steels with low chromium content. A small amount of 0.4% titanium addition significantly increases the ductility of 0.4%C–4%Cr-0.4Ti steel, leading to a good combination of high strength of 2270 MPa and high elongation of 18.3%. The substructure changes of the lath martensitic structure caused by the addition of titanium contribute to the brittle to ductile transition of FSWed medium-carbon steel with 4% chromium.